Pellet-type foams of non-crosslinked polypropylene resin having lower melting point and process and device for producing the same and molded foams therefrom

ABSTRACT

The present invention relates to a pellet-type non-crosslinked polypropylene foam having a melting point of 125 to 140□, and a process and device for producing said foam. Since the pellet-type foams of non-crosslinked polypropylene of the present invention has a lower melting point and a closed cell content of 80% or more, it is advantageous to mold such foams. The present invention also relates to an article molded from the above pellet-type non-crosslinked polypropylene foams.

TECHNICAL FIELD

[0001] The present invention relates to a pellet-type non-crosslinkedpolypropylene foam having a melting point of 125° C. to 140° C., aprocess for producing said foam, a device for realizing said process andan article molded from said foam.

BACKGROUND ART

[0002] Due to its excellent mechanical strength and cushioningproperties, a polypropylene resin foam is widely used as a packagingmaterial, a building material, a heat shield material and the like.However, since polypropylene has a high crystallinity and a low meltviscosity and is difficult to cross-link, it has hereto been quitedifficult to obtain a highly expanded product from the polypropylene.

[0003] U.S. Pat. No. 5,527,573 (issued Jun. 18, 1996) discloses extrudedclosed-cell polypropylene resin foam and several methods for producingsaid foam. The foam of the U.S. patent is in a plank form and has aminimal cross-sectional area of about 5×2.54 square centimeters, aminimal thickness of 12.7 millimeters and a density of about 5 poundsper a cubic foot. The form and properties make it difficult to mold thefoam into desired shaped articles. U.S. Pat. No. 6,051,617 (issued Apr.19, 2000) discloses a foamed polypropylene resin particle useful formolding a foamed, molded article and a method of preparing the same.However, the foamed polypropylene resin particle is prepared by graftinga vinyl comonomer to polypropylene resin particles to form the modifiedcopolymer resin particles and foaming the modified copolymer resinparticles. U.S. Pat. No. 6,077,875 (issued Jun. 20, 2000) disclosesfoamed and expanded beads of a polypropylene resin for molding preparedfrom a non-crosslinked propylene-ethylene random copolymer. The foamedbeads of the U.S. patent has an open cell content of at most 40%, mostpreferably 25% and has a melting point of at least 141° C.

DISCLOSURE OF INVENTION

[0004] A non-crosslinked polypropylene resin is advantageous as it canbe recycled and the pellet-type foam produced from the resin can beeasily molded. However, the pellet-type foam obtained through extrusionof the non-crosslinked polypropylene resin contains open cells to nosmall extent and thus is useless. For its practicability, thepellet-type foam must contain a great amount of closed cells formechanical strength. It is only JSP Corporation of Japan throughout theworld that successfully commercially produces a pellet-type foam from anon-crosslinked polypropylene resin. However, while it is recognizedthat the pellet-type non-crosslinked polypropylene foam having lowermelting point are highly valuable owing to its easy molding andexcellent recycling, a pellet-type non-crosslinked polypropylene foamhaving a melting point of 140° C. and less has not yet been produced.

[0005] Therefore, an object of the present invention is to provide apellet-type polypropylene resin foam with a melting point of 140° C. orless which is produced from a non-crosslinked polypropylene resin toensure a recyclability, has a high content of closed cells to provide asatisfactory mechanical strength and can be molded into various shapedpackaging materials, and a method for producing the same. The inventorshave successfully produced pellet-type foams of non-crosslinkedpolypropylene with a melting point of 125 to 140° C., which comprisesabout at least 40% of closed cells by providing a tandem extruder with aplurality of temperature zones having specifically varied temperatures,making the non-crosslinked polypropylene resin melt having a meltingpoint of 138 to 140° C. flow through the temperature zones, mechanicallyhomogenizing the polypropylene resin melt passed through such zones at alower temperature of 120 to 130° C., expanding the homogenized meltthrough a plurality of holes formed in the dies under pressure, andcutting expanded foams discharged from the holes of the dies.

[0006] In one aspect, the present invention provides a pellet-typenon-crosslinked polypropylene foam having a melting point of 125 to 140°C.

[0007] In another aspect, the present invention provides a method forpreparing a pellet-type non-crosslinked polypropylene foam having amelting point of 125 to 140° C., comprising steps of; (a) extruding anon-crosslinked polypropylene random copolymer having a melting point of138 to 140° C. through a tandem extruder; the tandem extruder consistingof the first extruder divided into the first temperature zone in which atemperature of 147 to 153° C. is set, the second temperature zone inwhich a temperature of 167 to 172° C. is set, the third temperature zonein which a temperature of 168 to 172° C. is set, the fourth temperaturezone in which a temperature of 218 to 225° C. is set, the fifthtemperature zone in which a temperature of 197 to 203° C. is set and thesixth temperature zone in which a temperature of 188 to 193° C. is set,the second extruder divided into the first temperature zone in which atemperature of 167 to 173° C. is set, the second temperature zone inwhich a temperature of 147 to 152° C. is set, the third temperature zonein which a temperature of 142 to 147° C. is set, the fourth temperaturezone in which a temperature of 137 to 141° C. is set, the fifthtemperature zone in which a temperature of 137 to 142° C. is set and thesixth temperature zone in which a temperature of 132 to 137° C. is set,and a guide, connecting the first extruder with the second extruder, inwhich a temperature of 248 to 255° C. is set; (b) compulsorily flowingthe extruded material at a temperature of 125 to 140° C. by pumping; (c)homogenizing the extruded material at a temperature of 120 to 130° C.;(d) expanding the homogenized material through a dies; and (e) cuttingthe expanded material to obtain the pellet-type foams.

[0008] In another aspect, the present invention provides a device forproducing a pellet-type non-crosslinked polypropylene foam having amelting point of 125 to 140° C.

[0009] In another aspect, the present invention provides articles moldedfrom pellet-type non-crosslinked polypropylene foams having a meltingpoint of 125 to 140° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

[0011]FIG. 1 is a view showing the overall structure of the device forproducing pellet-type foams of non-crosslinked polypropylene accordingto the present invention;

[0012]FIG. 2 is a plan view showing a cylinder of the first extrudershown in FIG. 1;

[0013]FIG. 3 is a sectional view taken along the line A-A in FIG. 2;

[0014]FIG. 4 is a schematic view showing the structure of the device forsupplying CO₂;

[0015]FIG. 5 is a sectional view showing the inner structure of thepumping part and the homogenizing part shown in FIG. 1;

[0016]FIG. 6A and FIG. 6B are front views showing each member of thehomogenizing part;

[0017]FIG. 7 is a plan view showing the structure of the die part shownin FIG. 1;

[0018]FIG. 8 is a sectional view taken along the line B-B in FIG. 7;

[0019]FIG. 9 is a sectional view taken along the line A-A in FIG. 8;

[0020]FIG. 10A is a side view showing a cylinder of the die part towhich a cooling device is mounted;

[0021]FIG. 10B is a front view of FIG. 10A;

[0022]FIG. 11 is a DSC curve of the pellet-type polypropylene foams(Example 1) prepared according to the present invention;

[0023]FIG. 12 is a DSC curve of the pellet-type polypropylene foams(Example 2) prepared according to the present invention;

[0024]FIG. 13 is a DSC curve of RP2400 (polypropylene-polyethylene (3%))copolymer used for preparing the pellet-type polypropylene foams of thepresent invention;

[0025]FIG. 14 shows the result of the FT-IR analysis of the pellet-typepolypropylene foams prepared according to the present invention;

[0026]FIG. 15 shows the result of the FT-IR analysis of RP2400(polypropylene-polyethylene (3%)) copolymer used for preparing thepellet-type polypropylene foams of the present invention;

[0027]FIG. 16A is a photograph taken by an optical microscope at ×100magnification and showing the pellet-type polypropylene foams preparedaccording to the present invention;

[0028]FIG. 16B is a photograph taken by an optical microscope at ×400magnification and showing the pellet-type polypropylene foams preparedaccording to the present invention;

[0029] Similar reference numbers refer to similar parts throughoutseveral views of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] Prior to the present invention, it has been impossible to preparepellet-type foams having a melting point of 140° C. or less. Purepolypropylene whose melting point is 138° C. could not be processed at atemperature of 138° C. or less, since it would be cured rapidly at itsmelting point or less. Thus, it has been also considered as beingimpossible to prepare pellet-type foams having a melting point of 140°C. or less.

[0031] However, the inventors have developed the pellet-typenon-crosslinked polypropylene foams which have a melting point of 140°C. or less and an open cell content of about 20% or less. Suchpellet-type foams have been developed by combination and application ofa number of discoveries. For example, the inventors have found that thecontent of open cells in foams can be remarkably decreased by using atandem extruding method as a basis, setting specific temperatureconditions for the first and second extruding process, and homogenizingthe melt resulting from the extruder at a lower temperature of 125 to130° C. Also, the inventors have found that only when the temperatureduring the extrusion and expansion of the non-crosslinked polypropyleneresin having a melting point of 138 to 140° C. is kept within a specifictemperature range, closed cells of 80% or more can be formed. Further,the inventor have found that a melting point of the foams prepared fromthe non-crosslinked polypropylene resin having a melting point of 138 to140° C. can be lowered to 138° C. or less by the specific workingconditions according to the present invention.

[0032] Now, the method for preparing the pellet type foams ofnon-crosslinked polypropylene according to the present invention will bedescribed in detail. The manufacturing method of the present inventioncomprises extruding, pumping, homogenizing, expanding and pelletizingsteps.

[0033] (1) Extruding

[0034] An extruding process according to the present invention can beperformed by using a tandem extruder that is commonly used and wellknown in the art for preparing foams as a basis or its variation.Materials used for preparing the pellet type foams of non-crosslinkedpolypropylene according to the present invention comprises anon-crosslinked polypropylene random copolymer having a melting point of138 to 140° C., a nucleating agent, a foaming agent and an additive, ifnecessary.

[0035] The basic resin of the present invention is the non-crosslinkedpropylene random copolymer having a melting point of 138 to 140° C.Examples of other comonomer copolymerizable with propylene includeethylene, 1-butene, 1-pentent and 1-hexene. The propylene randomcopolymer can be bipolymers such as propylene-ethylene random copolymeror propylene-butene random copolymer or terpolymers such aspropylene-ethylene-butene copolymer. The ratio of other comonomercomponent other than propylene in the copolymer is preferably 0.05 to10% by weight.

[0036] The nucleating agent functions to disperse a foaming agent andadjust the cell size of the foams. Examples of the nucleating agentwhich can be used in the present invention include, but are not limitedto, sodium bicarbonate, sodium carbonate, potassium bicarbonate,potassium carbonate, ammonium bicarbonate or ammonium carbonate. Sodiumbicarbonate is preferred. The more amount of the used nucleating agentis, the smaller the cell size of the foams is. On contrary, the lessamount of the used nucleating agent is, the larger the cell size of thefoams is. In the present invention, the nucleating agent of 0.1 to 0.4%based on the weight of a resin is used. When the amount of the usednucleating agent exceeds 0.4%, insufficient dispersion or agglomerationcan occur, and as a result, the cell grows larger than a predeterminedsize. On the contrary, when the amount of the used nucleating agent is0.1% or less, the nucleation activity is excessively weak, whereby thecell diameter can not be reduced.

[0037] As a usable foaming agent in the present invention, there areorganic foaming agents and inorganic foaming agents. Examples of theorganic foaming agent are aliphatic hydrocarbons such as propane,butane, hexane and heptane, alicyclic hydrocarbons such as cyclobutaneand cyclopentane, and halogenated hydrocarbons such aschlorofluorometnae, trifluorometane, 1,1-difluoroetnae,1,2,2,2-tetrafluoroetane, methyl chloride, ethyl chloride and methylenechloride. Also, usable organic foaming agents includedichlorotetrafluoroethane, trichlorotrifluoroethane,trichloromonofluoromethane, dichlorodifluoromethane,dichloromonofluoromethane and dibromotetrafluoroethane. Consideringforming workability, nontoxicity and flame retardancy, thesefluoro-chlorinated hydroncarbon are preferable. These organic foamingagents can be used alone or as a mixture of two or more thereof.Examples of the inorganic foaming agent include nitrogen, carbondioxide, argon and air. These inorganic foaming agents can be used aloneor as a mixture of two or more thereof. Also, any mixtures of randomlyselected two or more of the organic foaming agents and the inorganicfoaming argents can be used. The most preferred foaming argent is aninorganic foaming agent since they do not destroy an ozonosphere and areinexpensive. The used amount of the foaming agents depends upon theexpansion ratio of the foam pellet to be obtained and the type of theused resin and foaming agent. The amount of the foaming agent used inthe present invention is about 20% to 30% by weight, based on the weightof the resin.

[0038] In addition, various kinds of additives can be used. Examples ofsuch additives include an antioxidant, UV absorber, flame retardant,coloring agent, dye, metal deactivator and the like. These additives canbe used in an amount of 0.1 to 0.3% by weight, based on the weight ofthe copolymer resin. In a preferred embodiment of the present invention,paraffin wax is used. This helps the copolymer resin to flow and acts asan antistatic agent to remove static electricity of the copolymer resin.The used amount of paraffin wax is about 0.1% by weight.

[0039] The above materials used in the present invention are extruded bythe tandem extruder in which a specific physical condition is setaccording to the present invention and the process will be described asfollows. The tandem extruder is consisted of a first extruder, a secondextruder and a guide connecting the first and second extruders. A screwcompression ratio is generally 3:1. Usually, the inner diameter of acylinder of the first extruder is about 60 to 70 mm. The inner diameterof a cylinder of the second extruder is normally about 90 to 95 mm.

[0040] The first extruder is divided into six zones according to theirtemperatures, each zone corresponds to 300 to 400 LD. The six zones area first temperature zone of 148 to 153° C., a second temperature zone of167 to 172° C., a third temperature zone of 167 to 172° C., a fourthtemperature zone of 218 to 223° C., a fifth temperature zone of 197 to203° C., and a sixth temperature zone of 188 to 193° C.

[0041] The non-crosslinked polypropylene random copolymer resin and thenucleating agent are supplied at a constant speed through a hopper andthen melted in the first temperature zone in which a temperature of 148to 153° C. is set. The flowing speed can be adjusted by a rotating speedof the screw and is normally about 20 to 30 rpm. Such rotating speed ofthe screw determines an inflow speed of row material and a flow speed ofmelt. In this case, an inflow speed of the resin is about 25 km/hour.Though the copolymer resin and the nucleating agent can be suppliedthrough one hopper at the same time, it is preferable that they aresupplied independently via individual hoppers. The second and thirdtemperature zones are maintained at a temperature of 167 to 172° C.,other additives such as paraffin wax are introduced to an starting pointof the third temperature zone. The paraffin wax is pumped to beintroduced after it is melted. Also, the temperature of the fourthtemperature zone is set to 218 to 223° C. and a foaming agent issupplied to an starting point of the fourth temperature zone by pumpingoperation. The melt from the fourth temperature zone is passed throughthe fifth temperature zone in which a temperature of 218 to 223° C. isset and then introduced to the sixth temperature zone in which atemperature of 188 to 193° C. is set.

[0042] The melt from the sixth temperature zone of the first extruder isintroduced to the second extruder through the guide connecting the firstand second extruders. Here, the temperature of the guide is set to 248to 255° C. and LD of the guide is 300 to 400 mm.

[0043] The second extruder is also divided into six zones according to atemperature, each zone corresponds to 470 to 520 mm. The six zones are afirst temperature zone of 168 to 173° C., a second temperature zone of147 to 152° C., a third temperature zone of 143 to 147° C., a fourthtemperature zone of 137 to 142° C., a fifth temperature zone of 137 to142° C., and a sixth temperature zone of 132 to 137° C. A screw speed ofthe second extruder is normally 8 to 12 rpm.

[0044] Each zone in the first and second extruders can be maintained ata specifically set temperature by means of any one of water-cooled type,oil-cooled type and air-cooled type method. Among them, the water-cooledmethod that adjusts a temperature using water pressure is preferable.For example, a water cooled type device can be used, which has a coolingwater body which is formed in a hollow shape, so that it can be mountedaround the cylinder of the extruder and the cylinder is insertedtherein, and is provided with a cooling water passage formed integrallywith the body, so that cooling water is contacted directly with thesurface of the cylinder.

[0045] (2) Pumping

[0046] The sixth temperature zone of the second extruder can correspondto a flange for connecting a means for crushing the melt according to amethod of the present invention. Since the temperature of this flange ismaintained peculiarly at 132 to 137° C. by the present invention andthis temperature is lower than the melting point of 138° C. of thepolypropylene resin, an flowing speed of the melt can be remarkablylowered. Thus, there is need for forcibly flowing the melt so that itcan move smoothly. Such compulsory flow can be achieved by means of apump. At this time, the temperature is maintained at 125 to 138° C. bythe water-cooled type device.

[0047] (3) Homogenizing

[0048] According to the present invention, the melt transferred from theextruder by pumping at a temperature of 125 to 140° C. is homogenized ata temperature of 120 to 130° C. Here, to homogenize means that the meltis cut and ground at the same time in a manner of grinding stones. Also,through homogenization, the melt becomes to have a uniform temperaturein the inner portion and outer portion. In the course of homogenization,the temperature in the cylinder is kept at 120 to 130° C., preferably bya water cooling type or oil cooling type method, more preferably oilcooling type method. At this time, the pressure inside the cylinderreaches about 120 kg/cm².

[0049] (4) Expanding

[0050] The homogenized melt is expanded through dies. As describedabove, since a pressure inside the cylinder in which the melt arehomogenized reaches about 120 kgf/cm², a decompression means isinstalled at the dies to maintain a pressure of 0.3 to 0.7 kgf/cm². Thepolypropylene resin is expanded through holes of the dies. Here, adiameter of each hole is normally 0.5 to 1.0 mm, and an expansion ratiois normally five times of a diameter of the micro hole.

[0051] (5) Pelleting

[0052] The foams formed by expansion through the holes of the dies aredischarged and simultaneously cut by cutting members to produce pellettype foams.

[0053] A device for producing the pellet-type foams of non-crosslinkedpolypropylene which have a melting point of 125 to 140° C. according tothe present invention comprises a first extruder, a second extruderconnected to the first extruder, a pumping part connected to the secondextruder, a homogenizing part connected to the pumping part and a diespart connected to the homogenizing part

[0054] The first extruder comprises a cylindrical cylinder having ascrew shaft rotatably mounted therein, a driving means disposed at anend of the cylinder for rotating the screw shaft and a plurality ofcooling means and heating means disposed on an the circumference surfaceof the cylinder and is provided with inlets for supplying apolypropylene and a nucleating agent to the cylinder in an end portionof the cylinder near the driving means is and inlets for supplying anadditive such as an antistatic agent and a foaming agent in a propermiddle portion of the cylinder and an outlet at the other end portion ofthe cylinder. The polypropylene and a nucleating agent supplied to thecylinder via the inlet are transfered compulsorily toward the outlet bythe screw shaft which is rotated by the driving means.

[0055] The second extruder connected to the first extruder by a guidecomprises a cylinder to which the polypropylene melt discharged from thecylinder of the first extruder is supplied through the guide, and aplurality of cooling means and a heating means mounted to the outercircumference surface of the cylinder for adjusting a temperature of themelt in the cylinder.

[0056] The pumping part for moving compulsorily the polypropylene meltdischarged from the second extruder to the next device comprises acasing having an inner space to which the polypropylene melt dischargedfrom the cylinder of the second extruder is supplied, a pair of gearsrotatably installed in the casing, the gears being engaged with eachother, and a driving means for rotating the gears.

[0057] The homogenizing part comprises a cylindrical first housing towhich the polypropylene melt discharged from the cylinder of the pumpingpart is supplied, the first housing being rotatably installed rotatably,a driving means for rotating the first housing, a screw connected to adischarging end of the first housing, a second housing located on acircumference part of the screw, a frame mounted to an outercircumference of the second housing for forming an airtight spacebetween the second housing and the frame. A spiral space is formedbetween the screw and the second housing along the entire length of thescrew and the polypropylene melt discharged from the first housing isflow therethrough to be discharged and discharged to the outside. A heattransfer fluid oil flows in a space formed between the second housingand the frame to adjust a temperature of the polypropylene melt whichflows through the second housing.

[0058] The homogenizing part comprises a homogenizing means that crushesuniformly the polypropylene melt. The homogenizing means is composed ofa rotating plate rotatably mounted and a fixing plate disposed to becontacted with the rotating plate. The rotating plate is provided with aplurality of slits arranged radially and the fixing plate is providedwith a plurality of circular holes. The polypropylene melt introduced tothe homogenizing part arrives at the rotating plate and is cut by anedge of each opening of the rotating plate while it passes through therotating plate. Then, the cut polypropylene melt is ground by therotating plate in the space between the rotating plate and the fixingplate.

[0059] The ground polypropylene melt discharged from the homogenizingpart is supplied to the dies part including a discharging part, acutting part and a driving means, in which the expanded foams are cutinto a predetermined dimension.

[0060] In the present invention, the cooling means mounted to thecylinders of the first or second extruder has a closed casing throughwhich cooling water supplied from the outside flows. The cooling waterintroduced to the inside of the casing flows through the casing whilebeing in contact with the surface of the cylinder whereby thetemperature of the polypropylene melt that flows within the cylinder isreduced. The heating means disposed between two casings employs a heaterhaving a heating coil installed therein.

[0061] The cylinder of the first extruder is divided into sixtemperature zones according to a temperature condition which thepolypropylene melt flowing in the cylinder should satisfy. A temperatureof each zone is adjusted by the cooling means or heating means mountedon the outer circumference of the cylinder. A temperature of thepolypropylene melt should be kept at 147 to 153° C. in the firsttemperature zone, at 167 to 172° C. in the second temperature zone, at168 to 172° C. in the third temperature zone, at 218 to 225° C. in thefourth temperature zone, at 197 to 203° C. in the fifth temperature zoneand at 188 to 193° C. in the sixth temperature zone.

[0062] The cylinder of the second extruder is also divided into sixtemperature zones according to a temperature condition which thepolypropylene melt flowing in the cylinder should satisfy. A temperatureof each zone is adjusted by the cooling means or heating means mountedon an outer circumference of the cylinder. A temperature of thepolypropylene melt should be at 167 to 173° C. in the first temperaturezone, at 147 to 152° C. in the second temperature zone, at 142 to 147°C. in the third temperature zone, at 137 to 141° C. in the fourthtemperature zone, at 137 to 142° C. in the fifth temperature zone and at132 to 137° C. in the sixth temperature zone.

[0063] The guide connecting the first extruder and the second extrudershould be kept at a temperature of 248 to 255° C.

[0064] The two gears disposed in the inner space of the casing of thepumping part rotate in opposite directions from each other toward acenter of the inner space to make the polypropylene melt movecompulsorily to a next process position. Also, the first housing of thehomogenizing part is rotatably supported on supporting plates by aplurality of bearing blocks. A drive sprocket of a driving means isgeared with a driven sprocket fixed to an outer circumference surface ofthe first housing so that the first housing is rotated in response tothe operation of the driving means.

[0065] The discharging part of the dies part includes a hollow guide barand a cylinder located outside of the guide bar. The guide bar has aplurality of cavities formed on the outer circumference thereof in alongitudinal direction of the guide bar. The polypropylene meltsdischarged from the homogenizing part flows through each cavity. Aplurality of through holes are formed at portions of the cylindercorresponding to the cavities, respectively.

[0066] The cutting part of the dies part includes a supporting platelocated at a rear side of the discharging part and cutting member fixedto the supporting plate and movably located outside of the cylinder. Thecutting member is provided with a plurality of through holes formed atpositions corresponding to the plurality of through holes on thecylinder, respectively. The driving means comprises an eccentric cambeing able to rotate by a motor, a crank connected to the eccentric camand rotating in response to the rotation of the cam, and a powerconverting and transmitting means connected to the crank for convertinga rotating movement of the crank into a linear movement and transmittingthe linear movement to the supporting plate to which the cutting memberis fixed, whereby the cutting members are reciprocated along the outercircumference of the cylinder by the operation of the driving means andthe foams expanded from the through holes of the cylinder are cut byedges of the through holes of the cutting member.

[0067] Meanwhile, the cylinder provided with a plurality of groovesformed in a longitudinal direction on predetermined position thereof.Each groove has a reciprocating rod which is movably located therein andhas one end fixed to the supporting plate. A cutting member is fixed toeach reciprocating rod by a fixing means and is reciprocated on an outercircumference of the cylinder by the reciprocating rod which isreciprocated along each groove of the cylinder.

[0068] Now, a particular structure and operation of the device forproducing pellet-type foams according to the present invention will bedescribed in more detail taken in conjunction with the accompanyingdrawings.

[0069]FIG. 1 is a view showing an overall structure of a device forproducing the pellet-type foams according to the present invention. Thedevice for producing foams according to the present invention comprisesthe first extruder 100, the second extruder 200 connected to the firstextruder 100 via a guide 150, a pumping part 300 connected to the secondextruder 200, a homogenizing part 400 connected to the pumping part 300and a dies part 500 for forming a foam mixture discharged from thehomogenizing part 400 into pellet-type foams. Hereinafter, respectiveparts as mentioned above will be described individually.

[0070] A. First Extruder 100.

[0071]FIG. 2 is a plan view showing a cylinder of the first extruder 100shown in FIG. 1 and FIG. 3 is a sectional view taken along the line A-Ain FIG. 2. These figures show a structure of the first extruder 100. Thefirst extruder 100 comprises a cylindrical cylinder 101 having a certainlength, a driving means 102 installed at an end of the cylinder 101, ascrew shaft 103 mounted in the cylinder 101 and being able to rotate bythe driving means 102, and a cooling means 104 and heating means 105installed on the outer circumference surface of the cylinder 101.

[0072] At an end portion of the cylinder 101 near the driving means 102,entrances 101 a, 101 b (only one entrance 101 a is shown in FIG. 3 whichis a sectional view) for supplying a row polypropylene and a nucleatingagent (for example, sodium bicarbonate) to the cylinder 101,respectively, are formed. An exit 101 c is formed at another end portionof the cylinder 101. Also, at a middle portion of the cylinder 101, anentrance 101 d for supplying an antistatic agent (for example, paraffinwax) and an entrance 101 e for supplying a foaming agent (for example,LPG or CO₂) are formed.

[0073] On the other hand, an interior of the cylinder 101 is dividedinto six temperature zones according to temperature conditions of thepolypropylene that is supplied to the cylinder 101. The cooling means104 and the heating means 105 are mounted to the outer circumferencesurface of the cylinder 101 at positions corresponding to respectivetemperature zones for adjusting the temperature of the polypropylenemelt.

[0074]FIG. 2 is a plan view showing the cooling means 104 and theheating means 105 mounted on the outer circumference surface of thecylinder 101 of the first extruder 100 shown in FIG. 1. Now, the coolingmeans and heating means will be explained in detail referring to FIG. 3.

[0075] The cooling means 104 mounted on the outer circumference surfaceof the cylinder 101 corresponding to each zone has an airtight casingwith a donut shape, through which a cooling water supplied from anoutside flows. The cooling water introduced to the casing 104 flowsthrough the casing 104 while being in contact with the surface of thecylinder 101. Therefore, the temperature inside the cylinder 101, thatis, the temperature of the polypropylene melt that flows therein can belowered.

[0076] The heating means 105 mounted between two casings 104 (that is,cooling means) is a heater in which a heating coil is installed. Theheating means raise the temperature of the polypropylene melt which hasbeen already lowered by the cooling means 104 to a predeterminedtemperature.

[0077] The function of the first extruder 100 having a structure asdescribed above will be described with reference to respective drawings.As the driving means 102 is activated, polypropylene and a nucleatingagent are supplied to the cylinder 101 through the entrances 101 a, 101b, respectively. The screw shaft 103 is rotated in the cylinder 101 bythe action of the driving means 102 (of course, a rotating speed of thescrew shaft is lowered by a speed reducer, as compared with a rotatingspeed of the driving means), whereby the polypropylene and thenucleating agent supplied to the cylinder 101 are melted and mixed whilesimultaneously being moved compulsorily toward the other end of thecylinder 101.

[0078] In the process as described above, an antistatic agent and afoaming agent are supplied to the cylinder 101 through another entrances101 d, 101 e formed at a mid portion of the cylinder 101 to be mixedwith a polypropylene melt.

[0079] As described above, the cylinder 101 of the first extruder 100 isdivided into six temperature zones Z1 to Z6 according to the temperaturecondition of the polypropylene melt which flows therein as shown in FIG.2. Each of the temperature zones Z1 to Z6 has a length of about 300 to400 mm. In a preferred aspect, the cylinder 101 has an inner diameter of65 mm and LD of about 358 mm and the temperature condition and otherconditions of the polypropylene melt according to each temperature zoneZ1 to Z6 of the cylinder are as follows.

[0080] 1) First temperature zone Z1: A zone to which the polypropyleneand the nucleating agent are supplied, kept at a temperature of 150° C.,a length for which the above temperature is maintained, that is, alength of the first temperature zone Z1 is 360 mm.

[0081] 2) Second temperature zone Z2: The polypropylene melt flowingthrough the second temperature zone Z2 is maintained at a temperature of170° C.

[0082] 3) Third temperature zone Z3: The polypropylene melt flowingthrough the third temperature zone Z3 is maintained at a temperature of170° C. which is the same as in the second temperature zone Z2. Paraffinwax as an antistatic agent is supplied to the third temperature zone Z3.

[0083] 4) Fourth temperature zone Z4: The polypropylene melt flowingthrough the fourth temperature zone Z4 is maintained at a temperature of220° C. CO₂ or LPG as a foaming agent is supplied to the cylinder 101 atthe fourth temperature zone Z4.

[0084] 5) Fifth temperature zone Z5: The polypropylene melt flowingthrough the fifth temperature zone Z5 is maintained at a temperature of200° C.

[0085] 6) Sixth temperature zone Z6: The polypropylene melt ismaintained at a temperature of 190° C.

[0086] In order to meet the temperature conditions of the polypropylenemelt at respective temperature zones Z1 to Z6, the cooling means 104 andthe heating means 105 mounted on the temperature zones are properlyoperated. That is, the temperatures of the polypropylene melt at therespective temperature zones Z1 to Z6 are adjusted to meet the conditiondescribed above by controlling an amount and a temperature of a coolantsupplied to the casing of the cooling means 104 or a current applied tothe heating wire constituting the heating means 105 and a time to applya current.

[0087] On the other hand, in the case of using CO₂ as a foaming agentwhich is supplied to the cylinder 101 of the first extruder 100, anadditional device for supplying CO₂ is used. The device for supplyingCO₂ used in the present invention is as follow:

[0088] Referring to FIG. 4 which is a schematic view showing a structureof the device for supplying CO₂, the device 110 for supplying CO₂comprises a tank 110A for storing CO₂, an unit for vaporizing andfreezing 110B, an unit for supplying CO₂ 110C, an unit for stabilizing110D and a storage unit 110E. CO₂ stored in the storing CO₂ tank 110A istransferred to the unit 110B for vaporizing and freezing, in which it isconverted into the vapor phase. That is, in the course of passingthrough a refrigerator of the unit for vaporizing and freezing 110B, CO₂is gasified and vaporized, and then supplied to the unit 110D forstabilizing by a pump, the unit 110C. for supplying CO₂. In the unit110D for stabilizing, CO₂ in the form of vapor is converted to the gasphase. CO₂ in the gas phase is stored in the storage unit 110E. When aprocess is started, CO₂ stored in the storage unit 110E is supplied tothe first cylinder 101 of the first extruder 100 described above.

[0089] The polypropylene melt which meets the temperature conditions inthe temperature zones Z1 to Z6 is moved (moved by the screw shaft 103)toward an end of the cylinder 101 and then supplied to the secondextruder 200 through the guide 150. The polypropylene melt passingthrough the guide 150 is maintained at a temperature of 250° C.

[0090] B. Second Extruder 200

[0091] The second extruder 200 to which the polypropylene melt issupplied through the guide 150 has the same structure with the firstextruder 100. That is, the cylinder 201 constituting the second extruder200 is divided into six temperature zones according to the temperaturecondition of the polypropylene melt that flows in the cylinder 201. Acooling means and a heating means are mounted on the outer circumferencesurface of the cylinder 201 at positions corresponding to thetemperature zone for adjusting the temperature of the polypropylenemelt.

[0092] The cooling means and heating means also have the same structurewith the cooling means 104 and the heating means 105 mounted on theouter circumference surface of the cylinder 101 of the first extruder100 shown in FIG. 2. Therefore, the description on the structures of thecooling means and heating means is omitted.

[0093] The cylinder 201 of the second extruder 200 is divided into sixtemperature zones according to the temperature condition of thepolypropylene melt which flows therein. Each temperature zone has alength of about 470 to 520 mm. In a preferred aspect, the cylinder 201has an inner diameter of 90 mm and LD is about 495 mm. The temperatureconditions of the polypropylene melt in the cylinder 201 at thetemperature zones are as follows.

[0094] 1) First temperature zone (the entrance portlion): 170° C.

[0095] 2) Second temperature zone: 150° C.

[0096] 3) Third temperature zone: 145° C.

[0097] 4) Fourth and fifth temperature zones: 140° C.

[0098] 5) Sixth temperature zone (the exit portion): 135° C.

[0099] C. Pumping Part 300

[0100] The polypropylene melt at a temperature of 135° C. dischargedfrom the second extruder 200 is supplied to the pumping part 300. Sincethe melting point of the polypropylene is 138° C., the polypropylenemelt discharged from the second extruder 200 has been considerablyreduced in its flowing speed. In the present invention, the pumping part300 is used for moving compulsorily such polypropylene melt to asubsequent process.

[0101]FIG. 5 is a sectional view showing the inner structure of thepumping part 300 and homogenizing part 400 shown in FIG. 1. The leftside shows the inner structure of the pumping part 300 and the rightside shows an inner structure of the homogenizing part 400,respectively.

[0102] The pumping part 300 to which the polypropylene melt dischargedfrom the cylinder 201 of the second extruder 200 is supplied comprises acasing 301 having an inner space, a pair of gears 302A and 302B engagedwith each other and installed rotatably in the inner space of the casing301, and a driving means 303 for rotating the gears 302A and 302B. Anentrance portion of the casing 301 is connected to the cylinder 201 ofthe second extruder 200 by a flange 304.

[0103] The polypropylene melt discharged from the cylinder 201 of thesecond extruder 200 is supplied to the inner space of the casing 301 viathe entrance portion, and then discharged compulsorily to an exitportion by the two gears 302A and 302B which rotate in oppositedirections toward a center of the inner space of the casing. Thepolypropylene melt discharged from the cylinder 201 of the secondextruder 200 has a temperature of 135° C., and a considerably reducedfluidity. Therefore, the pumping part 300 serves to compulsorilytransfer the polypropylene melt to the next process.

[0104] D. Homogenizing Part 400

[0105] The homogenizing part 400 connected to the exit portion of thecasing 301 of the pumping part 300 is divided into a rotating part 400Aand a crushing part 400B. The rotating part 400A is composed of a firsthousing 401 of a hollow cylindrical shape, a driven sprocket 402 fixedto the outer surface of the first housing 401 and a driving means 404 towhich a driving sprocket 403 is fixed.

[0106] The first housing 401 is supported rotatably to support plates407 and 408 through a plurality of bearing blocks 405 and 406. The drivesprocket 403 of the driving means 404 is geared with the driven sprocket402 fixed to the outer surface of the first housing 401, whereby thefirst housing 401 is rotated in response to the operation of the drivingmeans 404. An end of the first housing 401 corresponds to the exitportion of the casing 301 of the pumping part 300 and thus, thepolypropylene melt discharged from the pumping part 300 flows in thefirst housing 401. In the first housing 401, the temperature of thepolypropylene melt are varied according to the location (that is, acentral portion and outer portion of the inner space of the firsthousing). However, since the polypropylene melt is mixed by the rotationmovement of the first housing 401, the whole polypropylene melt ismaintained constantly at a temperature. Here, since new polypropylenemelt is supplied compulsorily and continuously by the pumping part 400,the polypropylene melt is rotated and moved at the same time.

[0107] The crushing part 400B is composed of a screw 410 connected tothe exit portion of the first housing 401, a second housing 411 locatedon the outer circumference of the screw 410 and a frame 412 mounted onthe outer circumference of the second housing 411 for forming anairtight space between the second housing 411 and the frame 412. Thescrew 410 is a cylindrical member having a spiral with a certain depthformed on the outer circumference surface thereof and a spiral-shapedspace is formed between the screw 410 and the second housing 411 andextended to the entire length of the screw 410. Therefore, thepolypropylene melt discharged from the first housing 401 of the rotatingpart 400A under a high pressure, is moved along the spiral-shaped spacebetween the screw 410 and the second housing 411 and discharged out ofthe crushing part 400B.

[0108] Meanwhile, a heat transfer oil flows in the space formed betweenthe second housing 411 and the frame 412. That is, an inlet port 412Athrough which the heat transfer oil is supplied is formed at a side ofthe frame 412, an outlet 412B through which the heat transfer oil isdischarged is formed at the otner side of the frame 412. The heattransfer oil that introduced to the space between the second housing 411and the frame 412 via the inlet port 412A contacts directly with thesurface of the second housing 411 to adjust the temperature of thepolypropylene melt that flows in the second housing 411. The heattransfer oil that flows in the space between the second housing 411 andthe frame 412 to adjust the temperature of the polypropylene melt isdischarged via the outlet port 412B. Processes for inflow, adjustment oftemperature and discharge are proceeded continuously, whereby thetemperature of the polypropylene melt which flows in the spiral-shapedspace between the second housing 411 and the screw 410 is adjusted to apredetermined temperature.

[0109] At the rear end of the homogenizing part 400, that is, the exitpart of the second housing 411, a homogenizing means 450 is disposed forcrushing and homogenizing a mixture of the discharged polypropylene meltand a nucleating agent.

[0110]FIG. 6A and FIG. 6B are front views of a rotating plate 451 and afixing plate 452 constituting the homogenizing means 450, respectively.The rotating plate 451 and the fixing plate 452 have same shape and aremounted in a state that they are contacted with each other, providedthat the rotating plate 451 is mounted rotatably.

[0111] A plurality of openings 451A are formed radially on the rotatingplate 451, each opening 451A is slanted toward a center of the rotatingplate 451. Also, a plurality of circular holes 452A are formed on thefixing plate 452.

[0112] The polypropylene melt discharged from the second cylinder 411arrives at the rotating plate 451 which is rotating and passes througheach opening 451A formed on the rotating plate 451. At this point, thepolypropylene melt is cut by an edge of each opening 451A, whereby allpolypropylene melts are crushed homogeneously. The crushed polypropylenemelt is ground by the rotating plate 451 which rotates in the spacebetween the rotating plate 451 and the fixing plate 452 and dischargedthrough the holes 452A of the fixing plate 452.

[0113] E. Dies Part 500

[0114] The temperature-controlled polypropylene melts discharged fromthe homogenizing part 400 is supplied to the dies part 500 to producepellet-type foams.

[0115]FIG. 7 is a plan view showing a structure of the dies part shownin FIG. 1 and FIG. 8 is a sectional view taken along the line B-B inFIG. 7, and FIG. 9 is a sectional view taken along the line C-C in FIG.8. The dies part 500 is divided into a discharging part 500A, a cuttingpart 500B and a driving means 500C.

[0116] The discharging part 500A is composed of a hollow cylinder-shapedguide bar 501 and a cylinder 502 located outside the guide bar 501. Onthe outer circumference of the guide bar 501, a plurality of cavities501A are formed in a longitudinal direction of the guide bar 501. Thepolypropylene melt discharged from the homogenizing part 400 flows ineach cavity 501A. The plurality of through holes 502A are formed atportions of the cylinder 502 corresponding to the respective cavities501A.

[0117] The cutting part 500B is composed of a supporting plate 506located at the rear side of the discharging part 500A and a cuttingmember 503 fixed to the supporting plate 506. The cutting member 503 islocated movably on the outside of the cylinder 502 and has a pluralityof through holes 503A formed at positions corresponding to the pluralityof through holes 502A of the cylinder 502, as shown in FIG. 9. As shownin figures, a diameter of the through holes 502A formed on the cylinder502 is less than that of the through holes 503A formed on the cuttingmember 503. At an initial position, each through hole 502A of thecylinder 502 is located on a central portion of the correspondingthrough hole 503A of the cutting member 503.

[0118] Meanwhile, a plurality of grooves 502B are formed on the outercircumference of the cylinder 502 in a longitudinal direction andreciprocating rods 505 are located in the grooves 502B, respectively.The cutting member 503 is fixed to reciprocating rods 505 by fixingmeans such as a bolt, whereby the cutting member 503 is reciprocated onthe outer circumference of the cylinder 502 by the reciprocating rod 505which is reciprocated along the groove 502B of the cylinder 502.

[0119] The driving means 500C of the dies part 500 is composed of aneccentric cam 511 which is rotated by a motor 510, a crank 512 that isconnected to the eccentric cam 511 and rotated in response to therotation of the cam 511 and a power converting and transmitting means513 connected to the crank 512 for converting rotation movement of thecrank 512 to linear movement and transmitting the linear movement to thesupporting plate 506.

[0120] Once the eccentric cam 511 is rotated by the operation of themotor 510, the rotation movement of the crank 512 is converted to thelinear movement by the power converting and transmitting means 513, andthen transmitted to the supporting plate 506 to which ends ofreciprocating rods 505 are fixed. Therefore, the cutting member 503 isreciprocated along the outer circumference of the cylinder 502.

[0121] Meanwhile, the polypropylene melt discharged from thehomogenizing part 400 is injected under pressure to the plurality ofcavities 501A formed on the guide bar 501 and then expanded through thethrough holes 502A of the cylinder 502. At this time, when each throughhole 502A of the cylinder 502 corresponds to each through hole 503A ofthe cutting member 503 by the reciprocation of the cutting member 503 bythe operation of the driving means 510, the polypropylene melt passesthrough the through holes 502A and 503A and then is expanded to theoutside of the cutting member 503 with a certain length. Then, whenthrough holes 502A of the cylinder 502 are released from thecorresponding state with the through holes 503A of the cutting member503 by the movement of the cutting member 503, the expandedpolypropylene melt is cut by edges of the through holes 503A of thecutting member 503. Here, a dimension (length) of the cut foams isdetermined by a moving speed of the cutting members 503.

[0122] Thus, after the polypropylene melt is expanding through thethrough holes 502A of the cylinder 502 and cut by the cutting member503, the pellet-type foams are formed. A diameter of each through hole502A of the cylinder 502 is 0.7 mm and the expansion rate is about 5times of a diameter of the through hole. Also, the cutting member 503 isreciprocated at a speed of 600 revolutions per minute.

[0123] The polypropylene melt transferred from the crushing part 400Bare subjected under a pressure of 120 kgf/cm² in the dies part. If thepolypropylene melt in the dies is exposed directly to the atmosphere,most of the foams are open cells. In order to prevent production of opencells, in the present invention, a decompression means is installed atthe outside of the dies part.

[0124] In FIG. 7, an example of the decompression means installed at thedies part is shown. The decompression means 600 is a casing 601 forisolating the discharging part 500A and the cutting part 500B of thedies part 500 from the outside (the atmosphere). It is to be understoodthat a shape of the casing 601 is not limited. An entrance port 602through which air is introduced is formed at an side of the casing 601,an exit port 603 through which air is exhausted is formed at the otherside of the casing 601.

[0125] A temperature of the air which is supplied to the casing 601 isroom temperature or less and can be maintained by a cooling means (notshown). Also, it goes without saying that, in order to maintain properlya pressure in the casing 601 (for example, 0.8 Kgf/cm²), an amount ofthe air supplied to the casing 601 can be controlled by a pump (notshown). Meanwhile, the exit port 603 can be connected to a storage meansfor storing the prepared pellet-type foams along with discharged air.

[0126] A temperature in the cylinder 502 constituting the dies part 500is very high and therefore, the temperature of the cylinder 502 shouldbe properly maintained. For this, the present invention uses a coolingdevice 700 using the heat transfer oil, which is mounted on the diespart.

[0127]FIG. 10A is a side view showing a cylinder of the dies part towhich a cooling device is mounted and FIG. 10B is the front view of FIG.10A. The casing 601 of the decompression means 600 illustrated in FIG. 7is not shown for conveniences' sake.

[0128] The cooling device 700 used in the present invention comprises aring-type supplying pipe 701 located at the front of the cylinder 502,and through which heat transfer oil is supplied from the outside, aplurality of flowing pipes 702 connected to the supplying pipe 701 viaan entrance end 702A thereof and installed in the cylinder 502, and andischarging pipe 703 located at the front of the cylinder 502 andconnected to an exit end 702B of the flowing pipe 702.

[0129] The ring-type pipe 701 for supplying the heat transfer oil,located at the front of the cylinder 502, is connected at one end to acompulsory flowing means (not shown) such as a pump so that the heattransfer oil is supplied to the supplying pipe at a constant pressure.The heat transfer oil is injected to the plurality of flowing pipes 702through entrance ends 702A of the flowing pipes 702 connected to thesupplying pipe 701.

[0130] The plurality of the flowing pipes 702 disposed at a regularinterval on the entire outer circumference of the cylinder 502 areextended along the entire length of the cylinder 502, and each entranceend 702A and each exit end 702B of the flowing pipes 702 are exposed tothe front end of the cylinder 502. Therefore, the heat transfer oilsupplied through each entrance end 702A from the heat transfer oilsupplying pipe 701 flows through the flowing pipes 702 along the entirelength of the cylinder 502 (that is, after performing the heatexchange), and then is discharged via each exit end 702B.

[0131] The ring-type pipe 703 for discharging the heat transfer oillocated at the front of the cylinder 502 is connected at one end to theheat transfer oil storage tank (not shown). Thus, the heat transfer oilafter performing heat exchange with the cylinder 502 while flowingthrough the flowing pipes 702 flows into the heat transfer oildischarging pipe 703 through the exit end 702B and then is directed tothe heat transfer oil storage tank.

[0132] As described above, in the course of flowing through the heattransfer oil supplying pipe 701, the flowing pipes 702 installed in thecylinder 702 and the heat transfer oil discharging pipe 703, the heatexchange between the heat transfer oil and the cylinder 502 isaccomplished, whereby the cylinder can be constantly kept at a suitabletemperature for the production of foams.

[0133] Meanwhile, the supplying pipe 701 for supplying the heat transferoil to the flowing pipes 702 and the discharging pipe 703 to which theheat transfer oil from the flowing pipes 702 is supplied have a ringshape so that the heat transfer oil is simultaneously supplied to andreceived from the flowing pipes 702 mounted on the circumference of thecylinder 502 whose section is a circular shape. However, the shape ofthe supplying pipe 701 and the discharging pipe 703 is not limited to aring and can have a different shape, for example polygonal shape.

[0134] Now, the present invention will be described in more detail bythe following Examples. However, it is to be understood that thefollowing examples are presented to illustrate further various aspectsof the present invention, but are not intended to limit the scope of theinvention in any aspects.

EXAMPLE 1

[0135] In order to prepare the pellet-type foams of non-crosslinkedpolypropylene according to the invention, a tandem extruder having afirst extruder with the inner diameter of 65 mm and a second extruderwith the inner diameter of 900 mm was modified. A gear pump and a diespart were connected successively to the rear end of the second extruder.A homogenizing means as shown in FIG. 3 was installed at a positionwhere the melt is discharged from the gear pump, a compression means isprovided outside the dies. Through holes of the dies had a diameter of0.7 mm and a pressure in the dies was maintained at 0.5 kgf/cm². Arotating speed of the first extruder was set at 24 rpm and a rotatingspeed of the second extruder was set at 9 rpm.

[0136] 40 kg of random copolymer RP2400 (polypropylene-3 weight %ethylene; melting index 0.25; melting point 138° C.) commerciallyobtained from Yuhwa Korea Petrochemical Ind. Co., Ltd. and 800 g ofsodium carbonate commercially obtained from Keum Yang Co., Ltd. weresupplied to the extruder through respective hoppers. 300 g of paraffinwax M1 commercially obtained from Leochemical Co., Ltd (Kimhae, Korea)was supplied to the third temperature zone of the first extruder. 12 kgof LPG was supplied to the fourth temperature of the extruder by using ametering pump. Temperature conditions specified from the extruderthrough the homogenizing device are shown in Table 1, and LD of the eachtemperature zone was 360 mm. TABLE 1 Temperature condition TemperatureTemperature Device Zone No. (° C.) First 1^(st) 150 Extruder 2^(nd) 1703^(rd) 170 4^(th) 220 5^(th) 200 6^(th) 190 Guide* 250 Second 1^(st) 170Extruder 2^(nd) 150 3^(rd) 144 4^(th) 139 5^(th) 138 6^(th) 136 Gearpump 133 Homogenizing device 130

EXAMPLE 2

[0137] The temperature conditions in the device were set as described inTable 2 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 2 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 150 Extruder2^(nd) 170 3^(rd) 170 4^(th) 220 5^(th) 200 6^(th) 190 Guide* 250 Second1^(st) 170 Extruder 2^(nd) 150 3^(rd) 145 4^(th) 140 5^(th) 138 6^(th)135 Gear pump 130 Homogenizing device 125

EXAMPLE 3

[0138] The temperature conditions in the device were set as described inTable 3 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 3 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 147 Extruder2^(nd) 167 3^(rd) 168 4^(th) 218 5^(th) 202 6^(th) 188 Guide* 248 Second1^(st) 167 Extruder 2^(nd) 147 3^(rd) 142 4^(th) 137 5^(th) 137 6^(th)132 Gear pump 130 Homogenizing device 129

EXAMPLE 4

[0139] The temperature conditions in the device were set as described inTable 4 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 4 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 151 Extruder2^(nd) 170 3^(rd) 170 4^(th) 219 5^(th) 202 6^(th) 190 Guide* 252 Second1^(st) 170 Extruder 2^(nd) 150 3^(rd) 146 4^(th) 141 5^(th) 140 6^(th)135 Gear pump 130 Homogenizing device 127

EXAMPLE 5

[0140] The temperature conditions in the device were set as described inTable 5 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 5 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 153 Extruder2^(nd) 172 3^(rd) 172 4^(th) 225 5^(th) 203 6^(th) 193 Guide* 255 Second1^(st) 173 Extruder 2^(nd) 152 3^(rd) 147 4^(th) 141 5^(th) 142 6^(th)137 Gear pump 134 Homogenizing device 130

EXAMPLE 6

[0141] The temperature conditions in the device were set as described inTable 6 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 6 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 149 Extruder2^(nd) 170 3^(rd) 171 4^(th) 224 5^(th) 200 6^(th) 191 Guide* 250 Second1^(st) 170 Extruder 2^(nd) 150 3^(rd) 145 4^(th) 140 5^(th) 140 6^(th)135 Gear pump 134 Homogenizing device 130

EXAMPLE 7

[0142] The temperature conditions in the device were set as described inTable 7 and the pellet-type foams were prepared by using the sameprocedure and materials as in Example 1. TABLE 7 Temperature conditionTemperature Temperature Device Zone No. (° C.) First 1^(st) 150 Extruder2^(nd) 170 3^(rd) 170 4^(th) 220 5^(th) 200 6^(th) 190 Guide* 250 Second1^(st) 167 Extruder 2^(nd) 152 3^(rd) 142 4^(th) 141 5^(th) 137 6^(th)132 Gear pump 134 Homogenizing device 130

COMPARATIVE EXAMPLE 1

[0143] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except that the homogenizing device wasnot operated. The homogenizing device was maintained at a temperature of130° C. but was not activated. Therefore, the melt from the gear pumppassed through the temperature zone of 130° C. without homogenization tobe introduced to the dies.

COMPARATIVE EXAMPLE 2

[0144] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the first temperature zone ofthe first extruder, which was set to 146° C.

COMPARATIVE EXAMPLE 3

[0145] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the second temperature zone ofthe first extruder, which was set to 173° C.

COMPARATIVE EXAMPLE 4

[0146] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for third temperature zone of thefirst extruder, which was set to 173° C.

COMPARATIVE EXAMPLE 5

[0147] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fourth temperature zone ofthe first extruder, which was set to 226° C.

COMPARATIVE EXAMPLE 6

[0148] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fifth temperature zone ofthe first extruder, which was set to 205° C.

COMPARATIVE EXAMPLE 7

[0149] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the sixth temperature zone ofthe first extruder, which was set to 187° C.

COMPARATIVE EXAMPLE 8

[0150] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the guide, which was set to256° C.

COMPARATIVE EXAMPLE 9

[0151] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the first temperature zone ofthe second extruder, which was set to 174° C.

COMPARATIVE EXAMPLE 10

[0152] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the second temperature zone ofthe second extruder, which was set to 153° C.

COMPARATIVE EXAMPLE 11

[0153] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the third temperature zone ofthe second extruder, which was set to 148° C.

COMPARATIVE EXAMPLE 12

[0154] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fourth temperature zone ofthe second extruder, which was set to 142° C.

COMPARATIVE EXAMPLE 13

[0155] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fifth temperature zone ofthe second extruder, which was set to 143° C.

COMPARATIVE EXAMPLE 14

[0156] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the sixth temperature zone ofthe second extruder, which was set to 138° C.

COMPARATIVE EXAMPLE 15

[0157] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the gear pump, which was setto 141° C.

COMPARATIVE EXAMPLE 16

[0158] Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the homogenizing device, whichwas set to 131° C.

EXPERIMENTAL EXAMPLE 1

[0159] The pellet-type foams obtained from Example 1 and Example 2 weremeasured for DSC (differential scanning calorimeters; 10° C./min. to200° C., 50 cc/min. to N₂ purge) transition temperature according to thetest method KSM3050-2001. The results are shown in FIG. 11 and FIG. 12.As shown in the DSC curves of FIG. 11 and FIG. 12, the foams of Example1 and Example 2 had melting points of 137.62° C. and 128.38 ° C.,respectively, which are lower than the melting point of 138° C. of therandom copolymer (RP2400 (polypropylene-polyethylene(3%) randomcopolymer) used as the row material. RP2400 random copolymer as controlwas measured for the DSC transition temperature. The results are shownin FIG. 13.

EXPERIMENTAL EXAMPLE 2

[0160] The pellet-type foams obtained from Example 1 and RP2400(polypropylene-polyethylene(3%)) random copolymer as control weresubject to the elementary analysis. The analysis was performed by usingthe CE EA-1110 elementary analyzer. The results are shown in Table 8given below. TABLE 8 Results of the elementary analysis analysis itemSample C (%) H (%) N (%) RP2400 random copolymer 85.1 15.0 N.D.*Pellet-type foams of Example 1 84.2 14.8 0.5

EXPERIMENTAL EXAMPLE 3

[0161] The pellet-type foams obtained from Example 1 and RP2400(polypropylene-polyethylene(3%)) random copolymer as control weresubjected to the FT-IR analysis. The analysis was performed by using theBio-Rad Digilab FTS-165 FT-IR Spectrophotometer. The results are shownin FIG. 14 and FIG. 15, respectively. According to the results, it isnoted that the pellet-type foams obtained from Example 1 and RP2400random copolymer have a main component of polypropylene.

EXPERIMENTAL EXAMPLE 4

[0162] The open cell contents of the pellet-type foams prepared in theExamples and Comparative Examples were measured according to theprocedure C described in the ASTM D 2856-70. The results are shown inTable 9 given below. TABLE 9 Open cell No. content Example 1  6% 2 13% 312% 4 16% 5 14% 6  7% 7 12% Comparative 1 68% example 2 49% 3 52% 4 38%5 44% 6 39% 7 52% 8 40% 9 37% 10 36% 11 31% 12 53% 13 29% 14 28% 15 26%

EXPERIMENTAL EXAMPLE 5

[0163] Other physical properties of the foams prepared in Example 1 weremeasured according to a conventional method. The results are shown inTable 10 given below. TABLE 10 Item Foams of Example 1 Magnification  25 Appearance color Light ivory Cell Size  300 μm Compressive strength0.81 kgf/cm²

EXPERIMENTAL EXAMPLE 6

[0164] The pellet-type foams obtained from Example 1 was molded by usinga molder, 500GF 4, produced by Daekong Machinery Co., Ltd., with amolding pressure set to 2.5 kgf/cm² (temperature of about 138° C.), toproduce a well-molded article. Through production of such a well-moldedarticle, it is proved that the pellet-type foams according to thepresent invention are melted at a temperature of 138° C. to adhere toeach other, leading to sold bonding between foams.

EXPERIMENTAL EXAMPLE 7

[0165] The pellet-type foams obtained from Example 2 was molded by usinga molder, 500GF 4, produced by Daekong Machinery Co., Ltd., with amolding pressure set to 2.4 kgf/cm² (temperature of about 132° C.), toproduce a well-molded article. Through production of such well-moldedarticle, it is proved that the pellet-type foams according to thepresent invention are are melted at a temperature of 132° C. to adhereto each other, leading to solid bonding between foams.

[0166] From the above description, it will be appreciated that thepresent invention can be embodied as other specific forms by thoseskilled in the art without departing from the spirit and indispensablefeatures of the present invention. With regard this, it should beunderstood that the examples and experimental examples described abovehave been made by way of example and not as a limitation. It should beconstrued that all modification and change derived from the meaning andscope of the claims described below and equivalent rather than the abovedescription and equivalents thereof are intended to be included withinthe scope of this invention.

[0167] The pellet-type foams of non-crosslinked polypropylene accordingto the present invention has a closed cell content of 80% or more and amelting point of 125 to 140° C. so that the pellet-type foams of theinvention is much available in a standpoint of formation andregeneration.

What is claimed is:
 1. A pellet-type non-crosslinked polypropylene foamhaving a melting point of 125 to 140° C.
 2. The pellet-typenon-crosslinked polypropylene foam of claim 1, wherein the content ofclosed cells is 80% or more.
 3. The pellet-type non-crosslinkedpolypropylene foam of claims 1 or 2, wherein a melting point is from 125to 130° C.
 4. A method for preparing a pellet-type non-crosslinkedpolypropylene foam having a melting point of 125 to 140° C., comprisingsteps of; (a) extruding a non-crosslinked polypropylene random copolymerhaving a melting point of 138 to 140° C. through a tandem extruder; saidtandem extruder consisting of the first extruder divided into the firsttemperature zone in which a temperature of 147 to 153° C. is set, thesecond temperature zone in which a temperature of 167 to 172° C. is set,the third temperature zone in which a temperature of 168 to 172° C. isset, the fourth temperature zone in which a temperature of 218 to 225°C. is set, the fifth temperature zone in which a temperature of 197 to203° C. is set and the sixth temperature zone in which a temperature of188 to 193° C. is set, the second extruder divided into the firsttemperature zone in which a temperature of 167 to 173° C. is set, thesecond temperature zone in which a temperature of 147 to 152° C. is set,the third temperature zone in which a temperature of 142 to 147° C. isset, the fourth temperature zone in which a temperature of 137 to 141°C. is set, the fifth temperature zone in which a temperature of 137 to142° C. is set, and the sixth temperature zone in which a temperature of132 to 137° C. is set, and a guide connecting the first extruder and thesecond extruder with a temperature of 248 to 255° C.; (b) flowing theextruded material compulsorily by a pumping at a temperature of 125 to140° C.; (c) homogenizing the extruded material at a temperature of 120to 130° C.; (d) expanding the homogenized material through a dies; and(e) cutting the expanded material to obtain pellet-type foams.
 5. Themethod for preparing a pellet-type non-crosslinked polypropylene foam ofclaim 4, wherein said foam has closed cells of 80% or more.
 6. Themethod for preparing a pellet-type non-crosslinked polypropylene foam ofclaims 4 or 5, wherein said foam has a melting point of 125 to 130° C.7. An article molded from the foam of claim
 1. 8. A device for producinga pellet-type non-crosslinked polypropylene foam having a melting pointof 125 to 140° C., comprising the first extruder, the second extruderconnected to the first extruder, a pumping part connected to the secondextruder, a homogenizing part connected to the pumping part and a diespart connected to the homogenizing part, wherein the first extrudercomprises a cylindrical cylinder in which a screw shaft is mountedrotatably, a driving means installed at an end of the cylinder forrotating the screw shaft and a plurality of cooling means and heatingmeans mounted on an outer circumference of the cylinder, the cylinderhas entrances formed at an end portion thereof corresponding to thedriving means for supplying a row polypropylene and a nucleating agentthereto, an exit formed at another end portion thereof, entrances formedat a mid portion thereof for supplying an additive and a foaming agent,respectively, whereby the row material supplied to the cylinder via theentrances is flow compulsorily toward the exit by the screw shaftrotated according to the driving means; the second extruder comprises acylinder into which the polypropylene melt discharged from the cylinderof the first extruder is supplied through the guide, and a plurality ofcooling means and a heating means mounted to an outer circumferencesurface of the cylinder for adjusting a temperature of the polypropylenemelt in the cylinder.; the pumping part comprises a casing having aninner space into which the polypropylene melt discharged from thecylinder of the second extruder is supplied, a pair of gears installedrotatably and meshed with each other in the inner space of the casingand a driving means for rotating the gears, whereby the polypropylenemelt supplied therein is flow compulsorily; the homogenizing partcomprises a first cylindrical housing into which the polypropylene meltdischarged from the cylinder of the pumping part is supplied, the firsthousing being installed rotatably, a driving means for rotating thefirst housing, a screw connected to an exit end of the first housing, asecond housing located on an outer circumference of the screw, a framemounted to an outer circumference of the second housing for forming anairtight space between the second housing and the frame, and ahomogenizing means installed at a rear end of the second housing;wherein a spiral space is formed between the screw and the secondhousing along the entire length of the screw so that the polypropylenemelt discharged from the first housing is flow in the space formedbetween the screw and the second housing and discharged to an outside, aheat transfer fluid oil flows in a space formed between the secondhousing and the frame to adjust a temperature of the polypropylene meltwhich flows in the second housing, and the homogenizing means crushesthe polypropylene melt; the dies part into which the polypropylene meltdischarged from the homogenizing part is supplied is consisted of adischarging part, a cutting part and a driving means so that expandedfoams are cut at a certain size.
 9. The device for producing apellet-type foam of claim 8, wherein each cooling means mounted to thecylinders of the first and second extruders is a casing in which acooling water supplied from an outside flows, the cooling water suppliedto the casing is contacted with a surface of the cylinder and flow alongthe casing so that a temperature of the polypropylene melt that flows inthe cylinder becomes lower, each heating means mounted between twocasings is a heater in which a heating coil is installed.
 10. The devicefor producing a pellet-type foam of claim 8, wherein the cylinder of thefirst extruder is divided into six temperature zones according to thetemperature condition of the polypropylene melt supplied to thecylinder, a temperature of each zone is adjusted by the cooling meansand heating means mounted to an outer circumference of the cylinder, atemperature of the polypropylene melt is 147 to 153° C. at the firsttemperature zone, 167 to 172° C. at the-second temperature zone, 168 to172° C. at the third temperature zone, 218 to 225° C. at the fourthtemperature zone, 197 to 203° C. at the fifth temperature zone and 188to 193° C. at the sixth temperature zone.
 11. The device for producing apellet-type foam of claims 8 or 10, wherein a polypropylene and anucleating agent are supplied to the first temperature zone of thecylinder of the first extruder, an antistatic agent is supplied to thethird temperature zone, and a foaming agent is supplied to the fourthtemperature zone.
 12. The device for producing a pellet-type foam ofclaim 11, wherein the foaming agent to be supplied to the cylinder ofthe first extruder is CO₂ discharged from a device for supplying CO₂,the device for supplying CO₂ comprises a tank for storing CO₂, an unitfor vaporizing and freezing CO₂ connected to the tank, a stabilizingunit for transforming CO₂ supplied from the unit for vaporizing andfreezing into a vapor; and a storage unit for storing CO₂ supplied fromthe stabilizing unit to supply CO₂ into the first cylinder of the firstextruder.
 13. The device for producing a pellet-type foam of claim 8,wherein the cylinder of the second extruder is divided into sixtemperature zones according to the temperature condition of thepolypropylene melt supplied to the cylinder, a temperature of each zoneis adjusted by the cooling means and heating means mounted to an outercircumference of the cylinder, a temperature of the polypropylene meltis 167 to 173° C. at the first temperature zone, 147 to 152° C. at thesecond temperature zone, 142 to 147° C. at the third temperature zone,137 to 141° C. at the fourth temperature zone, 137 to 142° C. at thefifth temperature zone and 132 to 137° C. at the sixth temperature zone.14. The device for producing a pellet-type foam of claim 8, wherein thetwo gears mounted in the inner space of the casing of the pumping partare rotated in an opposite direction from each other toward a center ofthe inner space to make the polypropylene melt move compulsorily to anext process position.
 15. The device for producing a pellet-type foamof claim 8, wherein the first housing of the homogenizing part issupported rotatably to support plates through a plurality bearingblocks, a drive sprocket of the driving means is geared with a drivensprocket fixed to an outer circumference surface of the first housing sothat the first housing is rotated in response of an operation of thedriving means.
 16. The device for producing a pellet-type foam of claim8, wherein the homogenizing means of the homogenizing part is consistedof a rotating plate mounted rotatably and a fixing plate contacted withthe rotating plate, the rotating plate has a plurality of openingsarranged radially and the fixing plate has a plurality of circularholes, whereby after the supplied polypropylene melt is arrived at therotating plate and passed through each opening formed on the rotatingplate, the polypropylene melt is cut by an edge of each opening of therotating plate, and then the crushed polypropylene melt is groundbetween the rotating plate and the fixing plate by the rotating plate.17. The device for producing a pellet-type foam of claim 8, wherein thedischarging part of the dies part is consisted of a hollow guide bar anda cylinder located on an outside of the guide bar, the guide bar has aplurality of cavities formed on the outer circumference thereof in alongitudinal direction of the guide bar so that the polypropylene meltsdischarged from the homogenizing part flows in each cavity, a pluralityof through holes are formed at portions of the cylinder which arecorresponded to the cavities, respectively; the cutting part comprises asupporting plate located at a rear side of the discharging part andcutting members fixed to the supporting plate and located movably on anoutside of the cylinder, each cutting member has a plurality of throughholes formed at positions where are corresponded to the plurality ofthrough holes of the cylinder, respectively; and the driving meanscomprises an eccentric cam being able to rotate by a motor, a crankconnected to the eccentric cam and rotated in response to a rotation ofthe cam and a power converting and transmitting means connected to thecrank for converting a rotating movement of the crank into a linearmovement and transmitting the linear movement to the supporting plate towhich the cutting member is fixed; whereby each cutting member isreciprocated along the outer circumference of the cylinder according toan operation of the driving means so that the polypropylene foamsexpanded through each through hole of the cylinder is cut by edges ofthe through holes of the cutting member.
 18. The device for producing apellet-type foam of claim 17, wherein a diameter of each through holeformed on the cylinder is less than that of each through hole of thecutting member, each through hole of the cylinder is located on acentral portion of the corresponding through hole of the cutting memberat an initial position.
 19. The device for a producing pellet-type foamof claim 17, wherein the cylinder has a plurality of grooves are formedan outer circumference thereof in a longitudinal direction,reciprocating rods whose ends are fixed to the support plate are locatedmovably in the grooves, respectively, and each cutting member is fixedto each reciprocating rod by fixing means so that each cutting member isreciprocated on an outer circumference of the cylinder by the eachreciprocating rod which is reciprocated along each groove of thecylinder.
 20. The device for producing a pellet-type foam of claim 17,wherein the dies part further comprises a decompression means forpreventing a rapid change of a temperature and pressure to the expandedand extruded foams, the decompression means is a casing which is able toisolate the discharging part and cutting part from an outside(atmospheric), the casing has an entrance port through which an air isflow and an exit port through which an air is exhausted at both sides.21. The device for producing a pellet-type foam of claim 17, wherein thedies part further comprises a cooling device for cooling the cylinder,the cooling device comprises; a supplying pipe into which a heattransfer fluid oil is supplied from an outside, the supplying pipe beinglocated at a front of the cylinder; a plurality of flowing pipes havingentrance ends connected to the supplying pipe and mounted in thecylinder along the entire length of the cylinder; and a discharging pipebeing located at a front of the cylinder and connected to exit ends ofthe flowing pipes for receiving the heat transfer fluid which exchangeda heat with the cylinder.